for the record, here is my tier list, with my explanations. My experience is limited compared to many other players, but I'd like to think that my reasoning is sound and relatively un-biased (unlike hax. Seriously, has anyone seen his tier list? lol).
S-tier, most viable for winning tournaments: I think people have seen the success of marth and have tended to put marth above shiek. Though Marth definitely has the tools to beat anyone above him on the tier list, I think it is still clear that he does have some disadvantages that put him behind shiek, namely his laggy moves and lack of a projectile. Regardless, it is clear that these characters stand well above the rest of melee's cast.
A-tier, viable for winning tournaments: despite their disadvantages to characters in the S-tier, its apparent that a player can still out-play S-tier characters with relative ease depending on player skill. Only results from major tournaments really separate this tier from the above tier: rarely do you see a character from this tier in top 8 contention of a smash major (with the exception of jigglypuff). Despite having more top results, IC's cannot be above falcon or peach simply because those characters are truly unfavored match ups for IC's in a vacuum, and those characters are still only moderately disadvantaged against S-tier characters, as are IC's. Jiggs stands above the rest because of her exceptional aerial movement and aerial moves, as well as the strongest finisher in the game: rest. I really don't think enough top players are taking advantage of the potential rest has.
B1-tier, viable for winning tournaments, but clearly disadvantaged. Things get hairy in this tier. I think its safe to say pikachu is simply faster than samus, has amazing recovery options, has solid finishers, and has a projectile. I think the only thing samus has over pikachu is her projectiles and possibly her grab (though both of their grabs are awful which is probably the sole reason neither of them are in contention with the top 8). Its tough to say, but I do believe with more people following axe, we will see more people fleshing out pikachu and we may see a clearer distinction between the two. Either way, I think its clear that pikachu and samus stand above the rest of the mid tier characters with the exception of maybe luigi. I believe that doc has been placed much too highly on the previous tier list due to the skewed results of shroomed in the previous two or three years. Luigi, in my testing and observation, has proven to be more viable having slightly better recovery and significantly better movement with essentially equal moveset in comparison to doc. The number of top luigi players vs top doc players also attests to this: there are significantly less doc players than luigi players in the top 100.
B2-tier, viable for winning tournaments, but clearly disadvantaged. Again, things are hairy here, as top players of these characters are more inclined to skew results and may incorrectly portray their actual viability. I think ganon is still pretty strongly set on top here as he has devastatingly strong moves and is hard to KO with regular attacks due to his weight. Results from top ganon players seem to also show that he is positioned better than those below him on the tier list, but again, the data isn't very reliable for this tier so we can't definitely use that to say that ganon is better than everyone in this tier. Mario is strong, and quite possibly better than ganon (or even doc, IMO), but again, lack of top players and top results make it hard to say he is better than ganon. Y link and link are definitely placed correctly here having favorable match ups for anyone below them on the tier list, and definitely disadvantaged against anyone above them. I think link could possibly be better than young link granted he is heavier and has a significantly better up b, but the combos are definitely stronger for young link. Again, lack of top players really pushing these characters makes it cloudy in this regard. Admittedly, I have little to no experience with these characters and anyone past them on the tier list. As far as yoshi goes, I'll say it again: I think people are putting him much too high on their tier lists. Amsa has gotten away with the uniqueness of his character to make some results that really skew the actual viability of this character. All of the characters above him on the tier list have statistically and physically better movement and movesets. Amsa's outstanding results are much more a testament to his play skill than a testament to yoshi as a character, IMO.
C-tier, not likely to win tournaments. Not much to say here. G/W has great moves, much better than the following characters. Roy has great movement like marth but just really lacks the moveset, obviously. Everyone behind these two just really suffer from lack of good movement and good moves. Zelda's moves are all pretty strong, but really laggy. DK pretty much has all sub par moves but at least they're good enough to have moves above the bottom tier.
F-tier, not viable for winning tournaments. Pichu's nair and up smash are great, but everything else is pretty bad and of course the self-damaging clause is a real problem which puts him behind the rest of the cast. Mewtwo has a great nair, and thats pretty much it. I don't even know about kirby, but he's better than bowser simply because he's got slightly more options at his disposal in terms of moves and his recovery is of course great. Bowser literally has one saving grace to his competitive viability: up b. Outside that, it is really hard to land a move on an opponent without flat out predicting your opponent.